Dynamic area filtering for link-state routing protocols

ABSTRACT

In general, techniques are described for dynamically filtering, at area border routers (ABRs) of a multi-area autonomous system, routes to destinations external to an area by advertising to routers of the area only those routes associated with a destination address requested by at least one router of the area. In one example, a method includes receiving, by an ABR that borders a backbone area and a non-backbone area of a multi-area autonomous system that employs a hierarchical link state routing protocol to administratively group routers of the autonomous system into areas, a request message from the non-backbone area that requests the ABR to provide routing information associated with a service endpoint identifier (SEI) to the non-backbone area. The request message specifies the SEI. The method also includes sending, in response to receiving the request and by the ABR, the routing information associated with the SEI to the non-backbone area.

TECHNICAL FIELD

The disclosure relates to computer networks and, more specifically, torouting protocols used by computer networks.

BACKGROUND

A computer network is a collection of interconnected computing devicesthat exchange data and share resources. In a packet-based network, suchas the Internet, the computing devices communicate data by dividing thedata into small blocks called packets, which are individually routedacross the network from a source device to a destination device. Thedestination device extracts the data from the packets and assembles thedata into its original form.

Certain devices within the network, referred to as routers, use routingprotocols to exchange and accumulate topology information that describesthe network. This allows a router to construct its own routing topologydatabase of the network. Upon receiving an incoming data packet, therouter examines keying information within the packet and forwards thepacket in accordance with the topology information in the topologydatabase.

Many routing protocols use flooding-based distribution mechanisms toannounce topology information to routers within the network. Theserouting protocols typically rely on routing algorithms that require eachof the routers to have synchronized routing topology information. Thatis, flooding-based routing protocols require that all routers in therouting domain store, to the routers' respective topology databases, allof the routing information that has been distributed according to theprotocol. In this way, the routers are able to select routes that areconsistent and loop-free. Further, the ubiquity of the routinginformation allows the flooding process to be reliable, efficient andguaranteed to terminate. In operation, each router typically maintainsan internal topology (or “link-state”) database and scans the entiredatabase at a defined interval to generate and output link statemessages so as to synchronize the database to neighboring routers withinthe routing domain. In this way, link state is propagated across theentire routing domain and stored in full at each router within thedomain.

For example, Open Shortest Path First (OSPF) and Intermediate System toIntermediate System (IS-IS) routing protocols are link state protocolsthat use link state messages to ensure their routing topology issynchronized with respect to available interfaces, metrics and othervariables associated with network links. OSPF utilizes Link StateAdvertisements (LSAs) while IS-IS uses Link State Protocol Data Units(LSPs) to exchange information. A router generating a link state messagetypically floods the link state message throughout the network such thatevery other router receives the link state message. In networktopologies where routers are connected by point-to-point connections,each router floods link state messages to adjacent routers reachable oneach interface to ensure synchronization. For a network usingmulti-access media, such as an Ethernet network, the routers within thenetwork flood the link state messages to all other routers connected tothe network. In either case, the receiving routers construct andmaintain their own network topologies using the link informationreceived via the link state messages. IS-IS is specified in“Intermediate system to Intermediate system routing information exchangeprotocol for use in conjunction with the Protocol for providing theConnectionless-mode Network Service (ISO 8473),” ISO, ISO/IEC10589:2002, the entire contents of which is incorporated herein byreference. OSPF is described in “OSPF Version 2,” IETF Network WorkingGroup, Request for Comments 2828, April 1998, which is incorporated byreference in its entirety.

OSPF networks in an autonomous system (AS) may be administrativelygrouped into areas. Each area within an AS operates like an independentnetwork and has a unique 32-bit area identifier (area ID), whichfunctions like an Internet Protocol (IP) address. Area IDs are uniquenumeric identifiers, often expressed in dot-decimal notation, but theyare not IP addresses. Within an area, the topology database containsonly topology information for the area, LSAs are flooded only to routerswithin the area, and routers compute routes only within their respectiveareas. Sub-networks (“subnets”) are divided into other areas, which areconnected to form the whole of the autonomous system.

The central area of an AS, called the backbone area, has a specialfunction and is always assigned the area ID 0.0.0.0. (i.e., Area 0). Allother networks or areas in the AS are directly connected to the backbonearea by a router that has interfaces in more than one area. Theseconnecting routers are called area border routers (ABRs).

Because all areas are adjacent to the backbone area, an OSPF routersends all traffic not destined for the router's own area through thebackbone area. The ABRs in the backbone area are then responsible fortransmitting the traffic through to the appropriate ABR to thedestination area. The ABRs summarize the link-state records of each areaand advertise destination address summaries to neighboring areas. Theadvertisements contain the area ID of the area in which each destinationlies, so that packets are routed to the appropriate ABR.

An OSPF restriction requires all areas to be directly connected byphysical or virtual links to the backbone area so that packets can beproperly routed. All packets are routed first to the backbone area bydefault. Packets that are destined for an area other than the backbonearea (“non-backbone areas”) are then routed from the receiving backbonearea ABR to an appropriate backbone area ABR with an interface to thedestination area and then on to the remote host within the destinationarea.

SUMMARY

In general, techniques are described for dynamically filtering, at areaborder routers (ABRs) of a multi-area autonomous system, routes todestinations external to an area by advertising to routers of the areaonly those routes associated with a destination address requested by atleast one router of the area. In some examples, provider edge (PE)routers of an autonomous system provide reachability to services andmay, using a gateway protocol (e.g., Border Gateway Protocol (BGP))session, exchange service endpoint identifiers, such as InternetProtocol (IP) addresses for the PE routers, to announce theiravailability as a service endpoint to one or more services. Forinstance, a PE router that is a member of an area may receive, in a BGPsession, a service endpoint identifier for services reachable by aremote PE router that is a member of a different area. In order todetermine a path to the remote PE router and thereby reach the serviceendpoint represented by the remote PE router, the PE router requeststhat an ABR for the area provide to the PE router any routinginformation that has been received from the backbone area and that isassociated with the service endpoint identifier. As one example, aprefix that includes the service interfaces may have for a next hop (asspecified, e.g., by a BGP NEXT_HOP attribute) a service endpointidentifier that is a router identifier (e.g. a loopback IP address) forthe remote PE router. Accordingly, the PE router may request that theABR provide routing information associated with the router identifierfor the remote PE router.

The ABR for the area and having an interface to the PE router may beconfigured to filter, by default, all routes to destinations external tothe area such that the other routers in the area do not, according tothe default configuration, receive such routes from the ABR. However,upon receiving the request to provide to the PE router any routinginformation that has been received from the backbone area and that areassociated with the service endpoint identifier, the ABR installs apermit filter that directs the ABR to advertise such routing informationwithin the area that includes the PE router. Continuing the aboveexample, the ABR may advertise routing information associated with therouter identifier for the remote PE router. As a result, the PE routermay receive routing information for the remote PE router to enable thePE router to compute a Shortest-Path First calculation of a path to theremote PE router for forwarding IP or Label Distribution Protocol (LDP)traffic, for instance, toward the services reachable by the remote PErouter.

By employing this filtering model, area border routers of a multi-areaautonomous system may reduce a number of routes advertised intonon-backbone areas, which may improve route-convergence and lead toreduced resource consumption among the non-ABR area routers. Forexample, router for an area that is not an ABR may not receive allSummary and AS-external LSAs originated by routers external to the areaand may thereby avoid adding such LSAs to its topology database whilestill enabling receipt of area-external routes of interest upon request.The bandwidth usage of the area may also be reduced by avoiding floodingof irrelevant LSAs.

In one aspect, a method includes receiving, by a router, a serviceendpoint identifier for a remote router that provides reachability to aservice, wherein the router is logically located in a non-backbone areaof a multi-area autonomous system that employs a hierarchical link staterouting protocol to administratively group routers of the autonomoussystem into areas, and wherein the remote router is logically locatedexternal to the non-backbone area. The method also includes generating,by the router, a request message that requests an area border router forthe non-backbone area to provide routing information associated with theservice endpoint identifier to the non-backbone area in accordance withthe link state routing protocol. The method also includes sending, bythe router, the request message to the non-backbone area.

In another aspect, the method includes receiving, by an area borderrouter that borders a backbone area and a non-backbone area of amulti-area autonomous system that employs a hierarchical link staterouting protocol to administratively group routers of the autonomoussystem into areas, a request message from the non-backbone area thatrequests the area border router to provide routing informationassociated with a service endpoint identifier to the non-backbone area,wherein the request message specifies the service endpoint identifier.The method also includes sending, in response to receiving the requestand by the area border router, the routing information associated withthe service endpoint identifier to the non-backbone area.

In another aspect, a router includes a control unit comprising aprocessor, a network interface card, and a management interface executedby the control unit and configured to receive configuration informationthat configures the router as logically located in a non-backbone areaof a multi-area autonomous system that employs a hierarchical link staterouting protocol to administratively group routers of the autonomoussystem into areas. The router also includes a Border Gateway Protocol(BGP) module executed by the control unit and configured to receive aservice endpoint identifier for a remote router that providesreachability to a service, wherein the remote router is logicallylocated external to the non-backbone area. The router also includes alink state protocol module executed by the control unit and configuredto execute the hierarchical link state routing protocol, wherein thelink state protocol module is further configured to generate a requestmessage that requests an area border router for the non-backbone area toprovide routing information associated with the service endpointidentifier to the non-backbone area in accordance with the link staterouting protocol, and wherein the link state protocol module is furtherconfigured to send, via the network interface card, the request messageto the non-backbone area.

In another aspect, a router includes a control unit comprising aprocessor, a network interface card, and a management interface executedby the control unit and configured to receive configuration informationthat configured the router to operate as an area border router thatborders a backbone area and a non-backbone area of a multi-areaautonomous system that employs a hierarchical link state routingprotocol to administratively group routers of the autonomous system intoareas. The router also includes a link state protocol module executed bythe control unit and configured to receive a request message from thenon-backbone area that requests the area border router to providerouting information associated with a service endpoint identifier to thenon-backbone area, wherein the request message specifies the serviceendpoint identifier, wherein the link state protocol module is furtherconfigured to send, in response to receiving the request and via thenetwork interface card, the routing information associated with theservice endpoint identifier to the non-backbone area.

The details of one or more embodiments of the invention are set forth inthe accompanying drawings and the description below. Other features,objects, and advantages of the invention will be apparent from thedescription and drawings, and from the claims.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a block diagram illustrating an example multi-area autonomoussystem that employs a link state routing protocol and area border routerthat dynamically filters link state advertisements according totechniques described in this disclosure.

FIG. 2 is a block diagram illustrating example provider edge router thattriggers a request to an area border router for link state protocolrouting information in accordance with techniques described in thisdisclosure.

FIG. 3 is a block diagram illustrating an example router that floodslink state advertisements including routing information according to adynamic filter, in accordance with techniques described herein.

FIG. 4 is an example of a link state message for requesting link staterouting information associated with a service endpoint identifier, asdescribed herein.

FIG. 5 is a flowchart illustrating an example mode of operation for aprovider edge router to request routing information associated with aservice endpoint identifier according to techniques described in thisdisclosure.

FIG. 6 is a flowchart illustrating an example mode of operation for arouter operating as an area border router to dynamically filter routinginformation associated with a service endpoint identifier according totechniques described in this disclosure.

Like reference characters denote like elements throughout the figuresand text.

DETAILED DESCRIPTION

FIG. 1 is a block diagram illustrating an example multi-area autonomoussystem 2 that employs a link state routing protocol and area borderrouter 8 that dynamically filters link state advertisements according totechniques described in this disclosure. Example link state routingprotocols that may be employed by multi-area autonomous system 2 (“AS2”) as an Interior Gateway Protocol (IGP) include Open Shortest PathFirst (OSPF) and Intermediate System to Intermediate System (IS-IS). Inthe illustrated example, AS 2 includes three areas, backbone area 6(“backbone 6”) and non-backbone areas 4A-4B, administratively configuredto each operate as an independent network using the link state routingprotocol. In other words, each of the three areas 4A, 4B, and 6 runs itsown instance of the link-state routing protocol algorithm and thus eacharea may have a different link-state database and corresponding topologygraph. However, each of areas 4A, 4B and 6 may be described as within asingle overall IGP routing domain of autonomous system 2. Each of areas4A, 4B, and 6 may be associated with a different area identifier. Ininstances in which the link-state routing protocol is OSPF, the areaidentifier for backbone area 6 is the 32-bit integer 0.0.0.0 (asexpressed in dot-decimal notation).

Backbone 6 is the “central area” of the AS 2 and includes all of thearea border routers (ABRs) of AS 2-ABR 8 and ABR 9 in this example.Backbone 6 specially functions among the AS 2 areas to distributerouting information between non-backbone areas 4A, 4B. Backbone 6 isphysically and/or virtually contiguous.

Area border router 8 is attached to both area 4A and backbone 6 andcondenses topology information for area 4A for distribution to backbone6. Area border router 9 is attached to both area 4B and backbone 6 andcondenses topology information for area 4B for distribution to backbone6. Area border routers 8, 9 distribute condensed topology information torespective areas 4A, 4B. As described herein, connections betweenrouters that exchange link state messages may include point-to-point(P2P), broadcast multi-access (BMA), and/or non-broadcast multi-access(NBMA) connections.

Provider edge (PE) router 10A is an internal router of area 4A. That is,all networks directly connected to PE router 10A are part of area 4A. PErouter 10B is an internal router of area 4B. Any of PE routers 10A-10B(collectively, “PE routers 10”) may be an autonomous system boundaryrouter (ASBR). In some instances, PE router 10B may also oralternatively be an area boundary router. PE router 10B includes aninterface for a network configured with prefix 12, which may representan IPv4 or IPv6 prefix.

Each of PE routers 10 is deployed by a service provider or enterprise toprovide edge services to customers. PE routers 10 interface to customernetworks to offer support for the edge services, which may include Layer3 Virtual Private Networks (L3VPN), a Virtual Private Local Area Network(LAN) Service (VPLS), and/or Virtual LAN (VLAN). PE router 10B, forexample, may represent a service endpoint for any of these services andso provide reachability to customer networks connected to PE router 10Bthat engage the service. A service endpoint identifier for the serviceendpoint represented by PE router 10B may therefore be a routeridentifier for PE router 10B, such as a loopback IP address.

PE router 10B as an internal router of area 4B advertises to otherrouters of area 4B including ABR 9, using advertisement 18, routinginformation including a router identifier for PE router 10B and in somecases the states of the interfaces of PE router 10B connected to routersin area 4B. The PE router 10B router identifier may be a configured IPaddress, a unicast IP address configured on a loopback interface for therouter (i.e., a “loopback” or “local” IP address), or an IP address ofone of the interfaces of the router. Advertisement 18 may be flooded. Insome instances, advertisement 18 represents a Router-LSA (OSPF Type 1LSA). In general, a Router-LSA describes the collected states of arouter's interfaces to an area to which it is flooded. All OSPF LSAsbegin with a common 20-byte header that includes the router identifierfor the advertising router. In instances in which advertisement 18represents a Router-LSA, the advertising router may include the valuefor the router identifier for PE router 10B.

ABR 9 stores the routing information received in advertisement 18 to alink-state database for the link-state protocol. ABR 9 distributes,using advertisement 20, at least the router identifier of PE router 10Bin its link-state database to other routers of backbone 6 including ABR8, which stores the routing identifier of PE router 10B to its ownlink-state database. Advertisement 20 may be flooded by router 9 andreceiving routers out their interfaces. In some instances, advertisement20 represents a Summary-LSA (OSPF Type 3 LSA) generated by ABR 9 tospecify the routing identifier of PE router 10B as a reachable networkdestination. In general, a Summary-LSA is originated by an ABR foradvertisement within an area to describe destinations outside of thearea.

In this example, ABR 8 is pre-configured with a default filter offilters 14 that directs ABR 8 to eschew inter-area distribution ofrouting information to non-backbone routers in the general case. Thus,for example, although ABR 8 will distribute routing information receivedfrom PE 10A into backbone 6, ABR 8 in the default case does not exportrouting information received from backbone 6 into area 4A per thedefault route. In instances in which AS 2 implements OSPF, for example,the default filter of filters 14 may direct ABR 8 to not export anySummary-LSAs and AS-external LSAs received from backbone 6. AnAS-external LSA includes routing information that is imported at an ASBRinto OSPF from another routing process and that describes destinationsexternal to the AS. In this sense, area 4A is a stub area in thatAS-external LSAs are excluded from the area (as well as Summary-LSAs).

PE router 10A receives a service endpoint identifier for the PE router10B service endpoint by message 16, which is neither a Summary-LSA noran AS-external LSA (message 16 may in some cases be a special type ofSummary-LSA known as a default Summary-LSA, as described in furtherdetail below). Message 16 in the illustrated example originates from PErouter 10B. However, message 16 may in some cases be received from aroute reflector with which PE router 10A has a peering session. Forexample, message 16 may represent a Border Gateway Protocol (BGP) UPDATEmessage that includes a Network Layer Reachability Information (NLRI)for the service and that further includes a NEXT_HOP attribute thatspecifies a router identifier for PE router 10B in the form of aloopback (or “local”) IP address for the PE router 10B.

In some instances, message 16 represents a configuration message thatconfigures PE router 10A with configuration information that specifies arouter identifier for PE router 10B as a service endpoint identifier forone or more services. PE router 10A may be further configured with ormay receive from ABR 8 a default route specifying ABR 8 as theadvertising router (and therefore IGP next hop in this case) for andestination network that includes the router identifier for PE router10B. ABR 8 may provide the default route to PE router 10A using aSummary-LSA.

In accordance with techniques described herein, in the event PE router10A needs to determine a path through AS 2 to the service endpointrepresented by PE router 10B, PE router 10A sends request message 22 toABR 8 to request that ABR 8 provide any routing information receivedfrom backbone 6 and associated with the service endpoint identifier.Request message 22 may include the service endpoint identifier. Forexample, request message 22 may include the router identifier for PErouter 10B included in a NEXT_HOP attribute of a BGP UPDATE message thatincludes an NLRI for the service or as configured in PE router 10A asthe service endpoint for the service. Example instances of requestmessage 22 are described more fully below with respect to FIG. 4.

Area border router 8 receives request message 22 and, in response,advertises routing information from its link-state database andassociated with the service endpoint identifier within area 4A inresponse message 24, which PE router 10A receives. In some examples, ABR8 installs a filter to filters 14 to override the default filter fromfilters 14 in order to flood routing information associated with theservice endpoint identifier specified by the filter to area 4A. Theinstalled filter may represent a new type of opaque LSA received inrequest message 22 that indicates to ABR 8 to provide routinginformation associated with the service endpoint identifier area 4A.Again, an example of this new type of opaque LSA is described more fullywith respect to FIG. 4.

As a result, PE router 10A receives routing information for PE router10B to enable PE router 10A to compute a Shortest-Path First (SPF)calculation of a path to PE router 10B. PE router 10A may use the pathto forward IP or Label Distribution Protocol (LDP) traffic, forinstance, toward the services reachable by PE router 10B. For example,PE router 10A may use the path to request a label-switched path (LSP)label from ABR 8 according to LDP downstream on demand (DoD) andassociate a FEC with the label to reach the LDP FEC service endpoint atPE router 10B by an LSP. This technique may be applied to establish aVPLS service instance, for example.

By modifying the link-state push-based flooding mechanism in favor of apull-based mechanism by which ABR 8 filters area-external routinginformation for a link state protocol by default but responds torequests for routing information associated with a service endpointidentifier, the techniques may improve route-convergence times and leadto reduced resource consumption by PE router 10A and other internalrouters of area 4A while still permitting path computation toarea-external routers of interest.

FIG. 2 is a block diagram illustrating example provider edge router 30(“PE router 30”) that triggers a request to an area border router forlink state protocol routing information in accordance with techniquesdescribed in this disclosure. For purposes of illustration, PE router 30may be described below within the context of an example of networksystem 2 of FIG. 1 and may represent at least PE router 10A.

PE router 30 includes a control unit 31 and interface cards 48A-48N(“IFCs 48”) coupled to control unit 31 via internal links 54A-54N.Control unit 31 may include one or more processors (not shown in FIG. 2)that execute software instructions, such as those used to define asoftware or computer program, stored to a computer-readable storagemedium (again, not shown in FIG. 2), such as non-transitorycomputer-readable mediums including a storage device (e.g., a diskdrive, or an optical drive) or a memory (such as Flash memory, randomaccess memory or RAM) or any other type of volatile or non-volatilememory, that stores instructions to cause the one or more processors toperform the techniques described herein. Alternatively or additionally,control unit 31 may comprise dedicated hardware, such as one or moreintegrated circuits, one or more Application Specific IntegratedCircuits (ASICs), one or more Application Specific Special Processors(ASSPs), one or more Field Programmable Gate Arrays (FPGAs), or anycombination of one or more of the foregoing examples of dedicatedhardware, for performing the techniques described herein.

In this example, control unit 31 is divided into two logical or physical“planes” to include a first control or routing plane 32A (“control plane32A”) and a second data or forwarding plane 32B (“data plane 32B”). Thatis, control unit 31 implements two separate functionalities, e.g., therouting/control and forwarding/data functionalities, either logically,e.g., as separate software instances executing on the same set ofhardware components, or physically, e.g., as separate physical dedicatedhardware components that either statically implement the functionalityin hardware or dynamically execute software or a computer program toimplement the functionality in conjunction with hardware components.

Control plane 32A of control unit 31 executes the routing functionalityof PE router 30. In this respect, control plane 32A represents hardwareor a combination of hardware and software of control unit 31 thatimplements routing protocols link state protocol 36 and BGP 38. Controlplane 32A of control unit 31 executes BGP process 38 (hereinafter “BGP38”) to exchange, with BGP peers of PE router 30, inter-domain routinginformation stored in routing information base 40 (“RIB 40”). RIB 34 maystore active prefixes and their related attributes as received by BGP 38in BGP UPDATE messages. BGP peers of PE router 30 may include one ormore route reflectors.

Control plane 32A of control unit 31 also executes link-state protocolprocess 34 (hereinafter “link-state protocol 34”), which may representOSPF, IS-IS, or another hierarchical routing protocol that supportsdividing a network into separate areas. Link-state protocol 34 receiveslink-state advertisements (e.g., OSPF LSAs or IS-IS LSPs) from otherinternal routers of an area in which PE router 30 is logically locatedand from ABRs having an interface to the area. However, in accordancewith techniques described herein, ABRs for the area are configured toeschew flooding link-state advertisements into the area. A singleprocess or respective processes may execute link-state protocol 34 andBGP 38 using the operating environment provided by control unit 31.Because PE router 30 in this instance executes both link-state protocol34 and BGP 38, PE router 30 may be considered an ASBR and be configuredwith policies from the set of policies 42 that direct link-stateprotocol 34 to redistribute routes received via BGP 38 into link-statedatabase 36.

Link-state protocol 34 may resolve the topology defined by routinginformation in link-state database 36 to determine one or more routesthrough the link-state protocol 34 domain. Control plane 32A may thenupdate data plane 32B with these routes, where data plane 32B maintainsthese routes as forwarding information 70. Data plane 32B representshardware or a combination of hardware and software of control unit 31that forwards network traffic in accordance with forwarding information70. In some instances, data plane 32B may include one or more forwardingunits, such as packet forwarding engines (“PFEs”), which providehigh-speed forwarding of network traffic received by interface cards 48via inbound links 50A-50N to outbound links 52A-52N.

Control plane 32A further includes management interface 33 by which anetwork management system or in some instances an administrator using acommand line or graphical user interface, configures configurationdatabase to include a default route (e.g., a default-Summary LSA)specifying an ABR as a next hop for area-external destinations. Forexample, an administrator may configure PE router 30 operating as PErouter 10A with a default route specifying ABR 8 as a next hop forarea-external destinations including the routing identifier for PErouter 10B. An administrator may further configure PE router 30 with aservice definition that specifies a service endpoint identifierrepresented by a router that is external to the link-state protocol area(e.g., OSPF area) in which PE router 30 is logically located.

In some cases, BGP 38 may receive, in a BGP UPDATE message via anInterior BGP (IBGP) peering session with another router of thelink-state protocol 36 domain, a service endpoint identifier for aservice reachable by the identified service endpoint. The serviceendpoint identifier may be specified in the BGP UPDATE message as thenext hop for the service in the NEXT_HOP attribute, which an IBGPspeaker other than a route-reflector sets in accordance with a policy toits loopback address to enable other routers of the domain to resolvethe loopback address (other routers may be unable to resolve a next hopthat is an interface for a border router for another autonomous system).

In accordance with techniques described in this disclosure, controlplane 32A applies a policy from policies 42 that triggers a requestmessage 22 upon receipt (or in some cases storage to RIB 40) of the BGPUPDATE message that includes the service endpoint identifier. Link stateprotocol 36 generates request message 22 as a request for link staterouting information associated the service endpoint identifier andfloods request message 22 within the link state area for PE router 30for receipt by a connected ABR.

Subsequently, the connected ABR responds by flooding the requestedrouting information to the area for receipt by PE router 30. Link stateprotocol 36 receives the routing information associated with the serviceendpoint identifier and stores the routing information to link-statedatabase 34. The routing information may represent an OSPF Summary-LSAthat includes the longest prefix match for the service endpointidentifier in the link-state database of the ABR. Link state protocol 36resolves the routing topology represented by link state database 34 toidentify one or more routes to the service endpoint identifier forforwarding, e.g., LDP or IP traffic to the service endpoint.

BGP 38 may subsequently receive a BGP UPDATE message that includes aWithdrawn Routes field specifying a prefix for a route that is beingwithdrawn from service and that further includes the service endpointidentifier specified in a NEXT_HOP attribute. If RIB 40 no longerincludes any routes specifying the service endpoint identifier as a nexthop, a policy of policies 42 may trigger link state protocol 36 towithdraw the previous request made by PE router 30 by flooding requestmessage 22 to ABRs of the area.

Link state protocol 36 withdraws the previous request by generating andsending withdraw message 46 that requests ABRs to no longer send routinginformation associated with the service endpoint identifier. In somecases, withdraw message 46 represents a new type of LSA (correspondingto instances of request 22 that are the new type of LSA) that specifiesthe service endpoint identifier and further sets a link state age field(LS Age in OSPF LSAs) to a maximum age for the LSA. When received byABRs of the area, the ABRs will flush the LSA from their respectivedomains, thereby withdrawing the previous request made using request 22.

For cases in which an administrator configures PE router 30 with aservice definition that specifies a service endpoint identifierrepresented by a router that is external to the link-state protocol areain which PE router 30 is logically located, a policy from policies 42triggers the request 22 for routing information in a manner similar tothat described above with respect to the receipt of a service endpointidentifier via BGP.

FIG. 3 is a block diagram illustrating an example router 80 that floodslink state advertisements including routing information according to adynamic filter, in accordance with techniques described herein. Forpurposes of illustration, router 80 may be described below within thecontext of an example of network system 2 of FIG. 1 and may represent atleast ABR 8.

PE router 80 includes a control unit 81 and interface cards 95A-95N(“IFCs 95”) coupled to control unit 81 via internal links 94A-94N.Control unit 81, IFCs 95, and internal links 94A-94N may be structurallysimilar to control unit 31, IFCs 48, and links 54A-54N of PE router 30of FIG. 2.

In this example, control unit 81 is divided into control plane 82A anddata plane 82B. Control plane 82A of control unit 81 executes therouting functionality of router 80. In this respect, control plane 32Arepresents hardware or a combination of hardware and software of controlunit 31 that executes link state protocol processes 88A-88B (hereinafter“link-state protocols 88A-88B”), which may represent OSPF, IS-IS, oranother hierarchical routing protocol that supports dividing a networkinto separate areas. Link state protocol 88A receives/sends link-stateadvertisements (e.g., OSPF LSAs or IS-IS LSPs) from/to area borderrouters of a backbone area. Link state protocol 88B receives/sendslink-state advertisements (e.g., OSPF LSAs or IS-IS LSPs) from/to areaborder routers of a backbone area. Each of link state protocols 88A-88Bis associated with a different one of link state databases 86A-86B,though the link state protocols 88A-88B may import link stateadvertisements from either of link state databases 86A-86B. A singleprocess or respective processes may execute link state protocols 88A-88Busing the operating environment provided by control unit 81.

Link-state protocols 88A-88B may resolve topologies defined by routinginformation in link-state databases 86A-86B to determine one or moreroutes through the respective link-state protocol domains. Control plane32A may then update data plane 32B with these routes, where data plane32B maintains these routes as forwarding information 70. Router 80 inthis respect forwards packets from/to the non-backbone area to/from thebackbone area, for which router 80 has respective interfaces.

Data plane 82B represents hardware or a combination of hardware andsoftware of control unit 81 that forwards network traffic in accordancewith forwarding information 92. In some instances, data plane 82B mayinclude one or more forwarding units, such as packet forwarding engines(“PFEs”), which provide high-speed forwarding of network trafficreceived by interface cards 95 via inbound links 96A-96N to outboundlinks 97A-97N.

Control plane 82A further includes management interface 84 by which anetwork management system or in some instances an administrator using acommand line or graphical user interface, configures configurationdatabase. In accordance with techniques described herein, router 80 isconfigured so as to not, in the general case, flood link-stateadvertisements received from the backbone area into the non-backbonearea for which router 80 has respective interfaces. Specifically in thisexample, a filter from filters 14 configured in control unit 81prevents, in the general case, link state protocol 88B from importinglink state advertisements received by link state protocol 88A forflooding to the non-backbone area.

Upon receiving request message 22, link state protocol 88A adds (ormodifies the default) filter to filters 14 to allow flooding of routinginformation received from the backbone area and associated with theservice endpoint identifier specified in request message 22.Consequently, link state protocol 88A floods such routing information asis included in link state database 86B to the non-backbone area forreceipt by PE router 10A, for instance. Link state protocol 88A may insome instances query link state databases 86A-86B to identify aSummary-LSA that satisfies a longest prefix lookup query for the serviceendpoint identifier and flood the Summary-LSA to the non-backbone area.

In some instances, request message 22 may include a new type of LSA, inwhich case link state protocol 88A stores the LSA to link state database86A. This triggers link state protocol 88A to import the routinginformation associated with the specified service endpoint identifierfrom link state database 86B and flood it to the non-backbone area.

Link state protocol 88A may subsequently receive withdraw message 46that request router 80 to no longer flood routing information associatedwith the service endpoint identifier into the non-backbone area. In somecases, withdraw message 46 represents a new type of LSA (correspondingto instances of request 22 that are the new type of LSA) that specifiesthe service endpoint identifier and further sets a link state age field(LS Age in OSPF LSAs) to a maximum age for the LSA. In such cases, linkstate protocol 88A flushes the LSA from the non-backbone area (includingfrom link state database 86A) and no longer floods routing informationfor the service endpoint identifier into the non-backbone area torestore the general filtering case.

FIG. 4 is an example of a link state message 158 for requesting linkstate routing information associated with a service endpoint identifier,as described herein. Link state message 158 may represent any of requestmessages 22 of FIGS. 1-3. Link state message 158, as shown in theexample of FIG. 4, complies with the OSPF protocol in that it adheres tothe four byte width limitation specified by the OSPF protocol. That is,link state message 158 has a set of four-byte rows, as reflected in FIG.4 by the [0 . . . 31] bit range for each row shown at the top of linkstate message 158.

In addition, link state message 158 conforms in this example to a formatfor a class of OSPF LSAs referred to as “Opaque LSAs.” Opaque LSAs ingeneral, and link state message 158 in particular, includes a standardLSA header followed by application-specific information that may be useddirectly by OSPF or by another application. Additional details regardingOpaque LSAs are found in “The OSPF Opaque LSA Option,” RFC 5250, July2008, which is incorporated by reference herein.

As shown in FIG. 4, link state message 158 includes link state (LS) agefield 160A, options field 160B, and LS type field 160C. LS age field160A typically specifies the age of the link state message bearing linkstate message 158 in seconds and is used to distinguish between two linkstate messages specifying the same link state message sequence number intheir respective LS sequence number fields 160G. Options field 160B mayspecify which optional capabilities are associated with the link statemessage bearing link state message 158. LS type field 160C indicates theformat and function of the link state message bearing link state message158, i.e., the type of link state message. Opaque LSAs are Type 9, 10,and 11 link state advertisements. The value for LS type field 160C inthis example is Type 10 such that the flooding scope is area-local,although other types may be used.

The link-state identifier of link state message 158 is divided intoopaque type field 160D and opaque identifier 160E. Opaque type field160D may specify a registered or unregistered value for a “serviceendpoint identifier routing information request type” that indicates toreceiving routers that link state message 158 requests routing linkstate routing information associated with a service endpoint identifier.

Link state message 158 also includes advertising router field 160F, linkstate sequence number field 160G, link state checksum field 160H, andlength field 160I. Advertising router field 160F specifies the OSPFrouter identifier of the originator of link state message 158, such asPE router 10A of FIG. 1. LS sequence number field 160G is a signed32-bit integer used to detect old and duplicate link state messages. LSchecksum field 160H may be used to determine whether the link statemessage accompanying link state message 158 contains errors. Lengthfield 160I indicates the length of link state message 158. Althoughshown as containing header fields 160A-160I (“header fields 160”), linkstate message 158 may contain more or fewer header fields 160 in someexamples.

In accordance with techniques described herein, link state message 158further includes service endpoint identifier type-length-value (TLV)field 160J (“SEI TLV 160J”) in the opaque information field for the linkstate message 158. SEI TLV 160J includes a value for a service endpointidentifier, such as a router identifier for PE router 10B of FIG. 1 oranother router. The service endpoint identifier value may be an IPv4 orIPv6 loopback or interface address for the device that represents theservice endpoint. Although described as a TLV, SEI TLV field 160J may insome instances simply be the value of the service endpoint identifier,e.g., a 32-bit IPv4 address for a router.

An ABR that receives, on an interface for a link state protocol area,link state message 158 may extract the service endpoint identifierincluded in SEI TLV 160J. Based at least on the determination thatopaque type field 160D includes a value that indicates that link statemessage 158 is a request for routing link state routing informationassociated with a service endpoint identifier, the ABR floods suchrouting information for the extracted service endpoint identifier to thearea.

FIG. 5 is a flowchart illustrating an example mode of operation for aprovider edge router to request routing information associated with aservice endpoint identifier according to techniques described in thisdisclosure. The mode of operation is described with respect to PE router10A of FIG. 1.

PE router 10A receives, e.g., via a BGP peering session or aconfiguration message, a service endpoint identifier for a service thatis reachable by a remote PE router 10B that is logically locatedexternal to the link state protocol area in which PE router is logicallylocated (200). Although ABR 8 may be configured to prevent flooding ofSummary-LSAs and AS-external LSAs from backbone 6 into area 4A, PErouter 10A generates a request message 22 to request ABRs, e.g., ABR 8,to flood Summary-LSAs for the service endpoint identifier into area 4A(202). PE router 10A flood the request message 22 to area 4A for receiptby ABR 8 (204).

FIG. 6 is a flowchart illustrating an example mode of operation for arouter operating as an area border router to dynamically filter routinginformation associated with a service endpoint identifier according totechniques described in this disclosure. The mode of operation isdescribed with respect to ABR 8 of FIG. 1.

Initially, ABR 8 is pre-configured with a policy, or is in some casesstatically configured or otherwise pre-programmed with defaultoperation, to prevent flooding of routing information received frombackbone 6 into non-backbone area 4A (300). ABR 8 receives requestmessage 22 requesting ABR 8 to flood Summary-LSAs for a service endpointidentifier into area 4A (302). In response to the request message 22,ABR 8 adds a filter to filters 14 to dynamically enable flooding of anySummary-LSAs the service endpoint identifier into area 4A (304).

The techniques described herein may be implemented in hardware,software, firmware, or any combination thereof. Various featuresdescribed as modules, units or components may be implemented together inan integrated logic device or separately as discrete but interoperablelogic devices or other hardware devices. In some cases, various featuresof electronic circuitry may be implemented as one or more integratedcircuit devices, such as an integrated circuit chip or chipset.

If implemented in hardware, this disclosure may be directed to anapparatus such as a processor or an integrated circuit device, such asan integrated circuit chip or chipset. Alternatively or additionally, ifimplemented in software or firmware, the techniques may be realized atleast in part by a computer-readable data storage medium comprisinginstructions that, when executed, cause a processor to perform one ormore of the methods described above. For example, the computer-readabledata storage medium may store such instructions for execution by aprocessor.

A computer-readable medium may form part of a computer program product,which may include packaging materials. A computer-readable medium maycomprise a computer data storage medium such as random access memory(RAM), read-only memory (ROM), non-volatile random access memory(NVRAM), electrically erasable programmable read-only memory (EEPROM),Flash memory, magnetic or optical data storage media, and the like. Insome examples, an article of manufacture may comprise one or morecomputer-readable storage media.

In some examples, the computer-readable storage media may comprisenon-transitory media. The term “non-transitory” may indicate that thestorage medium is not embodied in a carrier wave or a propagated signal.In certain examples, a non-transitory storage medium may store data thatcan, over time, change (e.g., in RAM or cache).

The code or instructions may be software and/or firmware executed byprocessing circuitry including one or more processors, such as one ormore digital signal processors (DSPs), general purpose microprocessors,application-specific integrated circuits (ASICs), field-programmablegate arrays (FPGAs), or other equivalent integrated or discrete logiccircuitry. Accordingly, the term “processor,” as used herein may referto any of the foregoing structure or any other structure suitable forimplementation of the techniques described herein. In addition, in someaspects, functionality described in this disclosure may be providedwithin software modules or hardware modules.

Various embodiments have been described. These and other embodiments arewithin the scope of the following examples.

What is claimed is:
 1. A method comprising: receiving, by a router, aservice endpoint identifier for a remote router that providesreachability to a service, wherein the router is logically located in anon-backbone area of a multi-area autonomous system that employs ahierarchical link state routing protocol to administratively grouprouters of the autonomous system into areas, and wherein the remoterouter is logically located external to the non-backbone area; sending,by the router to the non-backbone area, a request message that requestsan area border router for the non-backbone area to reconfigure a filterassociated with the link state routing protocol to cause the area borderrouter to flood, in accordance with the link state routing protocol,link state information associated with the service endpoint identifierto the non-backbone area; and receiving, by the router, the link stateinformation flooded into the non-backbone area by the area border routerafter reconfiguring the filter, the link state information being usablefor determining a path to the remote router.
 2. The method of claim 1,wherein the request message comprises an Opaque Link State Advertisement(LSA) that conforms to an Open Shortest Path First (OSPF) routingprotocol and includes, in an opaque information field, the serviceendpoint identifier.
 3. The method of claim 2, wherein the Opaque LSAcomprises a first Opaque LSA, the method further comprising: sending, bythe router, a withdraw message to withdraw the request message, whereinthe withdraw message comprises a second Opaque LSA that includes a linkstate age field set to a maximum age for the second Opaque LSA.
 4. Themethod of claim 1, wherein the service endpoint identifier comprises oneof a router identifier, a loopback IPv4 address, or an interface IPv4address of the remote router.
 5. The method of claim 1, furthercomprising: establishing, by the router, a Border Gateway Protocol (BGP)session with a BGP peer, wherein receiving the service endpointidentifier comprises receiving, by the BGP session, a BGP UPDATE messagethat specifies the service endpoint identifier in a NEXT_HOP attributeof the BGP UPDATE message.
 6. The method of claim 5, wherein sending therequest message comprises sending the request message in response toreceiving the BGP UPDATE message.
 7. The method of claim 1, furthercomprising: after sending the request message, receiving, by the router,the routing information as one or more Link State Advertisements (LSAs)for the service endpoint identifier in accordance with the link staterouting protocol; computing, by the router, a path from the router tothe service endpoint identifier based at least on the LSAs; andforwarding packets destined for the service endpoint identifieraccording to the path.
 8. A router comprising: a control unit comprisinga processor; a network interface card; a management interface executedby the control unit and configured to receive configuration informationthat configures the router as logically located in a non-backbone areaof a multi-area autonomous system that employs a hierarchical link staterouting protocol to administratively group routers of the autonomoussystem into areas; a Border Gateway Protocol (BGP) module executed bythe control unit and configured to receive a service endpoint identifierfor a remote router that provides reachability to a service, wherein theremote router is logically located external to the non-backbone area;and a link state protocol module executed by the control unit andconfigured to execute the hierarchical link state routing protocol,wherein the link state protocol module is further configured to send arequest message that requests an area border router for the non-backbonearea to reconfigure a filter associated with the link state routingprotocol to cause the area border router to flood, in accordance withthe link state routing protocol, link state information associated withthe service endpoint identifier to the non-backbone area, and whereinthe link state protocol module is further configured to receive, by therouter, the link state information flooded into the non-backbone area bythe area border router after the filter is reconfigured, the link stateinformation being usable to determine a path to the remote router. 9.The router of claim 8, wherein the BGP module is further configured toestablish a BGP session with a BGP peer, and wherein to receive theservice endpoint identifier the BGP module is further configured toreceive, by the BGP session, a BGP UPDATE message that specifies theservice endpoint identifier in a NEXT_HOP attribute of the BGP UPDATEmessage.
 10. A non-transitory computer-readable medium comprisinginstructions for causing one or more programmable processors to:receive, by a router, a service endpoint identifier for a remote routerthat provides reachability to a service, wherein the router is logicallylocated in a non-backbone area of a multi-area autonomous system thatemploys a hierarchical link state routing protocol to administrativelygroup routers of the autonomous system into areas, and wherein theremote router is logically located external to the non-backbone area;send, by the router to the non-backbone area, a request message thatrequests an area border router for the non-backbone area to reconfigurea filter associated with the link state routing protocol to cause thearea border router to flood, in accordance with the link state routingprotocol, link state information associated with the service endpointidentifier to the non-backbone area; and receive, by the router, thelink state information flooded into the non-backbone area by the areaborder router after the filter is reconfigured, the link stateinformation being usable to determine a path to the remote router.